Experiments on rats show that following injection of S35-cysteine into the supraoptic nucleus, labeled oxytocin, vasopressin, and neurophysin appear first in the nucleus, then in the median eminence (1 hour), and finally in the posterior pituitary ( greater than 1.5 hour). The peptides and neurophysin are synthesized first as a protein precursor(s) (mw, 20,000 daltons) and the enzymatic degradation of the precursor appears to occur intra-granularly during axonal transport.